| BEN |
| BOTANICAL ELECTRONIC NEWS |
|---|
| ISSN 1188-603X |
| No. 365 August 16, 2006 | aceska@telus.net | Victoria, B.C. |
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The following text has been adapted from:
Introduction
Along the relatively dry eastern margin of Vancouver Island and in the southern Gulf Islands mixed grass and shrub ecosystems with varying densities of Garry oak (Quercus garryana) reach their northernmost limits (Stein 1990). They are characterized by a mosaic of disjunct nearly treeless prairies and meadows, open savannas, and closed woodlands. The understory in Garry oak communities supports a diverse range of grasses, herbs and shrubs that often are not found elsewhere in Canada (Fuchs 2001), including more than 100 "at risk" species - 23 of which have been identified as threatened or endangered throughout their global range (GOERT 2002).
The purpose of this study is to reconstruct the environmental history of a Garry oak ecosystem along a prairie- savanna- woodland gradient at Rocky Point, southern Vancouver Island, British Columbia. Specifically, we used a range of analytical tools, including a review of historical documents, aerial photographs, dendrochronology, and spatial analysis, in order to reconstruct fire history, stand composition, structure, and future trajectory. These analyses provide a context for assessing proposed management and restoration actions in Garry oak communities in coastal British Columbia.
Study Site
The Department of National Defence (DND) maintains and operates the Canadian Forces Ammunition Depot at Rocky Point, on southern Vancouver Island. This area is home to one of the largest remaining intact, and most intensively studied, Garry oak ecosystems on Vancouver Island.
At Rocky Point a representative Garry oak meadow that extended from open prairie, through savanna, and into an encroaching conifer forest was selected for analysis. The understory vegetation is very depauperate, and is composed primarily of introduced grasses. With the exception of Scotch broom (Cytisus scoparius) and gorse (Ulex europaeus) there are no shrubs in the area. Herbs are also rare within the area sampled, but does include camas (Camassia quamash), although only at very low densities.
Field Methods
The study site was subdivided into a grid of 10 m X 10 m grid plots for sampling. Within each of the resulting 90 plots, increment cores were collected from all trees greater than 10 cm diameter at breast height. Cores were taken as close as practical to ground level to accurately estimate establishment dates. All seedlings (individuals less than 1.5 m tall) and saplings (individuals more than 1.5 m tall, but less than 10 cm in diameter) were counted.
Results and Discussion
The oldest tree identified at Rocky Point established in ca. 1706. Following a long period without recruitment two distinct pulses in tree establishment occurred. Beginning in ca. 1850 Garry oak began establishing. Oak recruitment continued until ca. 1940, with a peak in establishment rates at ca. 1890.
Douglas-fir (Pseudotsuga menziesii) began to establish in ca. 1860, although in much smaller numbers than Garry oak, and continued until ca. 1920. The second pulse in recruitment began at ca. 1950 and has continued to the present. Except for a single oak tree it is characterized entirely by conifer recruitment.
This environmental history can be interpreted in the context of the anthropogenic history. Rocky Point was described by Captain W. Colquhoun Grant in ca. 1850, as "a fine open prairie extending nearly across to Becher Bay, interspersed with oak trees" (Grant 1857). That year, the Douglas Treaty was signed by the resident First Nations, who were subsequently moved out of the Metchosin Area (Weir 1983). Europeans began settling Rocky Point in the mid-1850s, and used it principally to raise horses, cattle, and sheep (Pethick 1971).
It seems likely, then, that the site was maintained in a largely tree-free condition by frequent burning by First Nations. Fires were likely excluded following European settlement, at which point oak began establishing in large numbers. It is not clear why recent establishment has consisted entirely of conifers, but possible explanations are considered below.
A spatial analysis of the locations of all seedlings, saplings, and trees sampled reveals three important insights into likely patterns of future stand development. First, conifer encroachment is proceeding largely from the north and west sides of the plot towards the centre. Second, Garry oak seedling presence is strongly associated with the absence of overstory conifers. Last, and perhaps most importantly, Garry oak seedling establishment does not appear to be a good indicator of subsequent overstory recruitment. Garry oak seedlings are very abundant at Rocky Point, representing 91 percent of the total seedling population, but saplings are virtually absent. Even assuming a very short understory residence time, this rate of recruitment is not sufficient to replace the current overstory population.
There are several possible controls on sapling recruitment at this site. Browsing by ungulates may be suppressing stem growth, causing the seedlings to allocate resources to below- ground production. Browsing intensity has likely been high at Rocky Point since 1850, due to high endemic deer populations, and use of the site by farmers to graze sheep and cattle until the mid 1990s. Competition with introduced annual grasses may also be inhibiting oak seedling survival. Gordon and Rice (2000) found that exotic grasses depleted soil moisture more rapidly than native perennial grasses, increasing stress and reducing survival rates of blue oak (Quercus douglasii) seedlings.
Fire exclusion at Rocky Point may also be restricting oak sapling recruitment. Episodic fire eliminates important competitors such as Douglas-fir and grand fir, allowing oak to gain a competitive advantage (Abrams 1992). Oak is well adapted to fire throughout its development, being able to sprout rapidly (Regan and Agee 2004), and developing a thick, fire-resistant bark over time.
Due to the longevity of Garry oak the current lack of recruitment at Rocky Point does not necessarily indicate that oak will ultimately disappear from the site. Given the episodic nature of oak regeneration, stand-wide disturbance every two- to three-hundred years could be at least theoretically sufficient to maintain a viable Garry oak population. In the context of the past several centuries, however, current conifer recruitment rates indicate that the stand at Rocky Point is undergoing dramatic changes.
Conclusions
Rocky Point has undergone a striking transformation from open prairie to savanna / woodland since the arrival of Europeans ca. 150 years ago. Management and remediation options are limited. Due to the use of Rocky Point as an ammunition depot by the DND, managers are understandably reluctant to use fire as a restoration tool. The mechanical removal of conifers would probably be sufficient to maintain Garry oak trees, but ultimately the causes of very low recruitment levels will need to be identified and addressed.
A related restoration dilemma also emerges from this analysis. At the time of European contact, Rocky Point was nearly treeless. The oak woodlands that have captured the attention of many conservation biologists are as much a consequence of fire exclusion as the current conifer encroachment. There may be no set of ecosystem processes or management practices that could maintain the ecosystem in anything resembling its current state. The recipe to make a Garry oak savanna may be to start with a nearly treeless prairie, and then exclude fire for a century. A corollary to this assessment is that because Garry oak savannas are non-equilibrium ecosystems, a large land base needs to be preserved in order to accommodate Garry oak associated ecosystems at various stages of development. Active management techniques are likely needed to promote desired conservation goals.
The USDA Forest Service recently launched the new Celebrating Wildflowers web site. The url is: http://www.fs.fed.us/wildflowers/
Celebrating Wildflowers is a series of events for people who are interested in native plants. Activities include wildflower walks, talks, festivals, slide shows, seminars, and planting events emphasizing the values and conservation of native plants.
The web site includes modules including Wildflower News, Just for Kids, Coloring Pages, Teacher Resources, Pollinators, Native gardening, Wildflower Ethics, Invasive Plants and Wildflower Links; listings of wildflower events, wildflower viewing areas and Plant of the Week. Of particular interest is a module entitled "Beauty of it All" a series of in-depth articles exploring the remarkable adaptations of wildflowers to diverse environments and specialized habitats. The inaugural story features the Lady Slipper Orchids.
This new web site is the gateway to an enormous amount of botanical information. The web site and the Wildflower Links are constantly being updated to keep information current and to add more botanically related pages. A number of other modules such as rare plants, native plant materials, ethnobotany, lichens, ferns and other botany subject areas are being developed and will be posted to the web site as they become finalized.
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